PROJECT SUMMARY Our recent transcriptome analysis of the brains of people with Down syndrome (DS), conducted from fetal stages to 40 years old, identified approximately 800 dysregulated genes across all chromosomes, each with specific temporal and regional profiles. These altered genes form co-expression networks, the most prominent of which indicates defective oligodendrocyte (OL) development and myelination. This finding is consistent with imaging studies demonstrating reduced white matter integrity in individuals with DS. In this collaborative study between the Haydar and Gallo labs, we will answer key questions regarding the timing and source of OL dysmaturation, and particularly whether cell-autonomous or non-cell autonomous mechanisms lead to altered cellular differentiation and myelination. The Aims of the project progress from defining the developmental time course of OL dysmaturation to comprehensive and integrated transplantation, behavioral and functional tests to evaluate the mechanism(s) of the defect. Whether these changes can be rescued by gene dosage normalization or by using newly identified pharmacological tools to prompt OL maturation will be studied in the last Aim.